An Agnostic is a person who believes that it is impossible to know if there is a deity or not. It is incorrectly considered the ultimate fence and neutrality between theists and atheists. Instead, it is a third side, stating that neither atheists or theists can be correct on the matter. Some forms of Agnosticism claim that there will never be a way to prove or disprove God, gods a life force, spirit, or ghost's existence. They are often lumped into the same category as Atheists, but that could be misleading given the existence of Theist Agnostics.
The weak agnostic is generally considered as fact by all rational people. No matter who you are, you cannot say that there is any evidence proving or suggesting the existence of (or lack thereof) a deity and be regarded as correct, considering that there is currently no proof whatsoever.
Weak Agnosticism isn't correct because a book said so (really, what book?), it's correct because it rejects belief as being fact. If it helps, consider the scientific method, but you cannot test and therefore analyze. Should your hypothesis be considered fact? Those that answer "no" are some form of Agnostic.
Types of Agnostics
Strong Agnostic: The belief in the existence of a god is ultimately unknowable.
Weak Agnostic: A Weak Agnostic knows that the existence of a god or not has not been proven as currently there is no evidence to come to a rational conclusion. This is the central idea of Agnosticism.
Apathetic Agnostic: Says that there is no proof or disproof of any deity and if it exists, it appears to not care about the universe, so its existence doesn't matter.
Agnostic Theism: Someone who does not claim to know for sure that a deity exists but chooses to believe in it regardless.
Agnostic Atheism: Someone who does not claim to know that a deity exists or not but has chosen not to believe in one.
Ignosticism—the view that a coherent definition of a god must be put forward before the question of the existence of a god can be meaningfully discussed. If the chosen definition is not coherent, the ignostic holds the noncognitivist view that the existence of a god is meaningless or empirically untestable.
Spiritual Agnosticism-the belief that whether or not a God exists or doesn't exist, it matters what you do...not what you believe.